I don't know about any interaction between marijuana and imatinib. This link may provide some useful information.
The issue with the line that marijuana helps cancer patients is a huge over-simplification. Marijuana typically contains around 100 component compounds, but there are two important ones in this area. Tetra Hydro Cannabinol (THC) is the stuff which makes you high. Cannabidiol (CBD) does not make you high, but reportedly has some beneficial effects, such as reducing nausea in those receiving chemo, and reducing muscle spasms in patients with cerebral palsy, and MS. The illegal drug suppliers are concentrating on high THC strains of the plant, for their profit. There is not enough work in big pharma on CBD - because it is a naturally occurring compound it can't be patented and therefore there is no huge profit to be made.